This invention relates to an electrochemical cell for measuring oxygen concentration in a gas. More particularly, it relates to an air/fuel ratio sensor with a stabilized zirconium dioxide solid electrolyte that is incorporated in the exhaust system of an internal combustion engine.
A zirconia oxygen concentration cell has already been considered for use as an air/fuel ratio sensor for an automotive internal combustion engine. Such a sensor is shown in FIG. 2 and is labeled as "Prior Art". This sensor is disclosed in allowed United States patent application Ser. No. 417,724, entitled "Air/Fuel Ratio Sensor", Burgett et al, filed Nov. 21, 1973, and assigned to the same assignee as the present invention.
In this type of sensor, ambient air, which is used as a reference gas, bathes a catalytic reference electrode. Exhaust gases flowing from the engine bathe a sensing catalytic electrode. The sensor output, E(volts), is produced by movement of oxygen ions between the two catalytic electrodes in accordance with the Nernst equation: ##EQU1## where R is the gas constant in joules/mole,
T is the absolute temperature, PA1 F is the Faraday constant in coulombs, PA1 P.sub.1 is the oxygen partial pressure of the air reference impinging the reference catalytic electrode, and PA1 P.sub.2 is the oxygen partial pressure of the exhaust gas impinging the sensing catalytic electrode.
In lean air/fuel ratio regions, the oxygen partial pressure (concentration) in the exhaust gases significantly increases. It can increase to almost the partial pressure of air. In such instance, the ratio of partial pressures between the reference gas and the exhaust gases approaches unity, whereupon the output voltage of the sensor drops nearly to zero. When the air/fuel ratio is rich, the electrode exposed to the exhaust gases senses a low oxygen partial pressure. In this latter instance, the ratio of partial pressures becomes greater than one, whereupon there is a representative significant increase in output voltage of the sensor.
The oxygen concentration in the exhaust gases is a direct function of the air/fuel ratio entering the intake manifold of an internal combustion engine. Zirconia sensors have a unique characteristic in that they exhibit a drastic change in output voltage at about stoichiometric air/fuel ratios. It is generally accepted that improved engine performance is obtained at stoichiometric air/fuel ratios. Consequently, the output from the zirconia sensor can be used to control air/fuel metering systems at the carburetor and continuously adjust them to maintain the engine air/fuel mixtures at stoichiometric ratios.
The prior art type of device shown in FIG. 2 has provided extremely satisfactory results. However, two platinum catalyzed electrodes are used, and platinum is costly. Moreover, all of the air/fuel ratio sensors heretofore proposed have required a separate reference atmosphere, for example ambient air, for one of the catalytic electrodes. The reference gas catalytic electrode must be imperviously sealed from the exhaust gases, and vice versa. This requires somewhat complex design of the sensor housing, adds to cost, can decrease reliability, and so forth. Moreover, the zirconia element may crack under unusually adverse conditions. A crack may allow exhaust gases to mix with the reference gas, and consequently reduce the output of the sensor.
I have found a new and simple zirconia sensor construction that does not require two catalytic electrodes, does not require a reference gas, does not require electrode sealing and whose operation is not deleteriously affected by leaks through cracks in the zirconia body.